JP5061078B2 - Casting simulation method and program thereof - Google Patents

Description

  The present invention relates to a casting simulation method capable of predicting cracks in a mold such as a core and a program thereof.

A member having a complicated shape is often produced by casting, and in addition to a die-casting method using a die, a pressure casting method using a sand mold, a gravity casting method, and the like are often used. In particular, when a hollow member having a hollow portion is manufactured, a collapsible core in which casting sand or the like is hardened with a binder is disposed in the outer mold and is often cast.
By the way, it is preferable that such a mold does not cause cracks or the like during casting. If cracks such as cracks occur in the mold during casting, the molten metal flows into that part, and the molten metal solidifies to form excess burrs and the like, leading to reduced casting yield and increased deburring man-hours. .
JP 2007-167893 A

Therefore, if it is possible to predict in advance by simulation the occurrence of mold cracks and the locations where such burrs are generated, the results will be reflected in the mold design to produce good castings with few burrs. Is possible.
However, what has been analyzed by the simulation so far is not a crack of the mold but a crack of the casting that occurs before and after the completion of solidification, as described in Patent Document 1 above. And it has not been proposed until now that the present inventors have investigated to predict the crack of the mold itself by simulation.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a casting simulation method and a program thereof capable of accurately predicting cracks in a mold.

As a result of extensive research and trial and error, the present inventor has conducted trial and error, and as a result, cracks in the mold (particularly the core) do not occur after the mold cavity has been filled with the molten metal, Newly found to occur during filling. For this reason, focusing on before and after completion of solidification, the conventional simulation method for predicting cracks in castings has led to the finding that cracks in molds cannot be accurately predicted.
Therefore, the present inventor considered that the mold crack could be accurately predicted by simulating the crack of the mold while the molten metal was filled into the cavity, and the actual casting result was obtained. We succeeded in obtaining simulation results that match By developing this result, the present inventor has completed various inventions described below.

<Casting simulation method>
(1) That is, in the casting simulation method of the present invention, a model setting step for setting a mold model in which a mold constituting a cavity filled with a molten metal is modeled on a coordinate system, and in the set mold model A casting simulation method comprising a filling analysis step for sequentially calculating a state in which a molten metal is filled into a cavity,
The model setting step includes a model formation step of forming the mold model by positioning it on a coordinate system, an element creation step of creating a large number of minute elements by finely dividing an area in the formed mold model, An element defining step of defining a microelement corresponding to a cavity region in the mold model as a cavity element among the created microelements and defining a microelement corresponding to the mold region in the mold model as a mold element; Have
The filling analysis step includes a filling analysis step for calculating a filling process in which the molten metal is sequentially filled into the cavity elements, and a thermal stress analysis step for sequentially calculating thermal stress acting on at least a part of the mold elements. A crack determination step for determining whether or not the calculated thermal stress exceeds at least a crack stress that is an index for determining the occurrence of mold cracks, and at least the presence or absence of mold cracks can be predicted. And

(2) In the casting simulation method of the present invention, the thermal stress acting on the mold is analyzed in the filling analysis process for simulating the filling of the molten metal into the cavity (after the filling analysis step or in parallel with the filling analysis step). Is done. That is, first, in the thermal stress analysis step, thermal stress acting on each part (mold element) of the mold (mold) is sequentially calculated. Next, in the crack determination step, the thermal stress thus calculated is compared with a crack stress that serves as an index for determining whether or not the mold is cracked. Specifically, it is determined whether or not the thermal stress exceeds at least the cracking stress. Based on the determination result, a simulation result in which mold cracks occur at least in the mold part (mold region) where the thermal stress exceeds the crack stress can be predicted. Here, “whether or not the thermal stress exceeds at least the cracking stress” may be determined whether the thermal stress exceeds the cracking stress or may be determined if the thermal stress is equal to or higher than the cracking stress. Means that.

  Further, each step in the filling analysis process according to the present invention may be performed by repeating a predetermined number of times even if a plurality of steps are sequentially performed one by one (one by one) and repeated a predetermined number of times. The processing of the next step may be performed after the processing of that step is completed.

(3) By the way, the reason why mold cracks hardly occur after completion of filling of the molten metal, and the reason why it occurs during the filling process is considered as follows.
First, until the filling of the molten metal is started and completed, the mold is formed with a high-temperature part that is in contact with the high-temperature molten metal and suddenly becomes high temperature, and a low-temperature part that is not in contact with the molten metal. . Although it depends on the material of the mold, the temperature difference between the high temperature part and the low temperature part becomes very large if it is viewed in a short time during the filling process. As a result, a large difference in thermal expansion occurs between the high temperature part and the low temperature part in a very short time, and the thermal expansion is partially constrained, so that a large thermal stress acts on the mold. It seems that the mold was damaged (die cracking).

  According to the casting simulation of the present invention, such a situation can be reproduced on a computer, so that it is possible to predict a mold crack that matches a real mold crack with high accuracy.

(4) Incidentally, the low temperature portion includes not only the unfilled portion of the molten metal but also the inside of the mold of the portion filled with the molten metal (high temperature portion). Particularly in the case of a sand mold or the like, there is a gap between the particles and the thermal conductivity is poor, and when the constituent particles are ceramic inorganic particles, the thermal conductivity of the particles themselves is also poor. For this reason, within a short time during which the molten metal is poured and filled, a large temperature difference tends to occur between the surface layer portion of the filled portion in contact with the molten metal and the inside of the mold. Therefore, the low temperature part may occur not only in the unfilled portion of the molten metal but also in the filled portion of the molten metal.

  In addition, since the thermal stress referred to in the present invention is largely caused by the temperature distribution difference (thermal expansion difference) of the mold, for example, the “thermal stress” acting on the mold referred to in the present invention is defined as “temperature”, and “thermal stress” The “analysis step” can be paraphrased as a “temperature analysis step” in which the thermal stress is calculated based on the temperature. However, since the thermal stress acting on the mold can be affected by the constraint conditions of the mold and the molten metal pressure acting on the mold, an analysis method suitable for the casting method is preferably selected.

<Casting simulation program>
The present invention is not limited to the above-described “method” invention, but is understood as a “product” invention. That is, the present invention may be a casting simulation program characterized in that the above-described casting simulation method is executed by causing a computer to function.

  If the program is not grasped as “thing”, it can be grasped as a computer-readable recording medium on which the program is recorded. Furthermore, it can also be grasped as a casting simulation apparatus that executes the program. In these cases, “process” and “step” in the present invention may be read as “means”. For example, the model setting process may be replaced with model setting means, and the filling analysis process may be replaced with a filling analysis means.

<Others>
The metal composition constituting the “metal melt” referred to in the present specification is not limited, and examples thereof include various cast irons, cast steels, Al alloys, and Mg alloys.
Further, the “mold” includes various molds such as a so-called sand mold and a mold, and the shape and material thereof are not limited. For example, a mold made of a tool rope or the like is used for casting a light alloy. In casting of cast iron or the like, a sand mold in which cast sand is hardened with a binder is used. In the case of a hollow casting, the outer mold may be a mold and the core may be a sand mold.

  Incidentally, a mold in which particles are hardened with a binder such as a sand mold (hereinafter referred to as “caking type”) is, for example, powder molding in which a mixed powder in which constituent particles and a binder are uniformly kneaded is pressed into a desired shape. The body is obtained as a fired fired body. As raw materials to be used as the “particles”, inorganic particles such as silica sand, mullite, and alumina, or metal particles such as iron powder and Al (Al alloy) powder can be used. As the binder, a binder, a resin adhesive, a thermosetting resin (for example, phenol resin), a gas curable resin, or the like can be used. However, in the present invention, the type of mold, method, casting method and the like are not limited. However, mold cracking is relatively likely to occur when a mold is made by solidifying particles with a binder (particle type, sand type), particularly when it is a core. Therefore, the casting simulation method of the present invention is particularly effective in predicting mold cracking of such a mold (particularly a core).

  The present invention will be described in more detail with reference to embodiments of the invention. In the present specification, the “casting simulation method” of the present invention will be mainly described for the sake of convenience. However, the contents described in the present specification can be appropriately applied to a program (casting simulation program) and the like for executing the same. Which embodiment is the best depends on the target, required performance, and the like.

  The casting simulation method of the present invention includes a model setting process and a filling analysis process. However, since the present invention is a casting simulation, it is more preferable to add a solidification analysis step to these steps as shown in FIG. Below, each of these processes is demonstrated one by one.

<Model setting process>
The model setting step is a step of setting a mold model obtained by modeling a mold constituting a cavity filled with molten metal on a coordinate system. The model setting process includes, for example, a model formation step, an element creation step, an element definition step, and an inflow position setting step as shown in FIG.

(A) Model formation step The model formation step is a step of forming a mold model by positioning the shape of the mold on the coordinate system. When the mold is composed of a plurality of mold members, the mold model need not have the shape of each mold member individually positioned on the coordinate system, and the shape of the entire mold is positioned on the coordinate system. It is enough if it is done.

  Positioning the shape (particularly the outer shape) of the mold or the mold member on the coordinate system can be performed by converting the shape into numerical data. If the shape of the mold already exists as CAD data or the like, it is efficient to use it. Of course, the acquisition of numerical data is not limited to CAD data, but can also be obtained using CAE, a coagulation analysis simulation device, or the like. Furthermore, it is also possible to convert the shape of an actual mold or casting produced using a three-dimensional scanner or the like into numerical data and form a mold model from the numerical data.

  Incidentally, the coordinate system to be used is generally a Cartesian coordinate system, but is not limited thereto, and an appropriate coordinate system such as a cylindrical coordinate system or a spherical coordinate system may be selected according to the shape of the mold and the analysis method.

(B) Element creation step The element creation step is a step of creating a large number of minute elements obtained by dividing the region in the mold model formed in the model formation step. That is, it is a step of subdividing the type model positioned on the coordinate system into microelements for analysis. By this step, the space on the coordinate system partitioned by the mold model is divided into a large number of microelements made of a polyhedron. The number of divisions or the division width may be appropriately set in consideration of analysis accuracy, calculation time, and the like.

  The divided shape of the elements is arbitrary, and may be an orthogonal hexahedron as employed in the finite difference method, or may be a polyhedron corresponding to the mold shape as in the finite element method. However, when the finite difference method is used, there is an advantage that the division into small elements is easy and the analysis becomes mathematically simple. Further, the sizes of the microelements do not have to be the same, and it is possible to improve the analysis accuracy by setting the microelements finely locally. For example, when it is desired to predict a portion where mold cracks are a problem with high accuracy, it is preferable to set fine elements in such a portion finely.

  It is not always necessary to divide the microelements over the entire region (space) in the mold model, but it is necessary to form a range necessary for the filling analysis step or the solidification analysis step, that is, a cavity region and its boundary. It is also possible to do only in the range.

(C) Element definition step The element definition step defines at least a microelement corresponding to the cavity region in the mold model among the many microelements created in the element creation step as a cavity element and also molds in the mold model This is a step of defining a microelement corresponding to a region as a mold element. In this step, for example, a cavity element that is not filled with molten metal is defined as a void element, a cavity element that is filled with molten metal is defined as a filled element, or the interface between the cavity element and the mold element is a surface element. Or is defined. In short, the element definition step is a step of defining attributes of each microelement for the filling analysis process or the solidification analysis process.

  This element definition step is a step performed after the above-described element creation step, but it is not necessary to perform the element definition step after all the minute elements have been created in the element creation step. That is, every time one or more microelements are created in the element creation step, an element definition step for defining the attribute may be performed, and this operation may be repeated.

  In this specification, the “mold region” is a region where the mold itself is formed, and is a portion where the molten metal is not injected. The “cavity region” is a portion where a molten metal is poured and a molded product such as a casting is finally formed. Further, the interface between the “mold region” and the “cavity region” may be used as the boundary region.

  A method for defining each microelement as a cavity element or a mold element is not particularly limited. An example is shown in FIGS. In the following, for convenience, the mold model and the minute elements will be described two-dimensionally, but the same is true in the case of three-dimensional.

  FIG. 3 shows a case where orthogonal coordinates are employed and the microelement 1 is a square (three-dimensionally a polyhedral shape such as a rectangular parallelepiped or a cube). The wavy line on the coordinates is the boundary of the cavity. When the center of gravity 2 of each microelement 1 exists in the mold area (shaded portion), the microelement 1 is defined as a mold element, and when it exists in the cavity area, the microelement is defined as a cavity element.

  FIG. 4 shows a state where each microelement 1 is defined as a mold element or a cavity element. A case where the center of gravity 2 exists in the mold region is represented by a white circle ◯, and a case where the center of gravity 2 exists in the cavity region is represented by ●. In addition, it is good to prescribe | regulate that the microelement 1 which does not correspond to any of a type | mold area | region and a cavity area | region does not become a calculation load.

(D) Inflow position setting step The inflow position setting step is a step of setting the inflow position of the molten metal to the selected cavity element selected from the cavity elements near the position where the molten metal is poured into the cavity. The selected cavity element is defined, for example, near the pouring gate for gravity casting and near the outlet of the plunger for pressure casting. There may be one or more selected cavity elements. And according to the actual pouring situation, pressurization situation, etc. of a molten metal, it is good to give a weighting coefficient to each of a plurality of selected cavity elements.

<Filling analysis process>
As shown in FIG. 6, the filling analysis process includes a filling analysis step, a thermal stress analysis step, and a crack determination step.
(1) Filling analysis step In the filling analysis step, the filling process in which the molten metal is filled into the cavity in the set mold model is sequentially calculated. As a result, the physical behavior of the molten metal injected into the cavity is analyzed in units of minute elements every minute time.

A specific calculation method is not particularly limited, and a known or commonly used method can be used. For example, VOF (Volume of Fluid), SOLA, FAN, and their improved calculation methods can be used. As basic equations used in these analyses, as shown in FIG. 5, (1-1) continuity equation, (1-2) Navier-Stokes equation, (1-3) VOF (interface There is a formula of (tracking).
Therefore, the filling analysis step includes small steps such as a flow velocity / pressure calculation step, a molten metal moving step, and an element flag changing step as shown in FIG.

(A) Flow velocity / pressure calculation step In the flow velocity / pressure calculation step, the flow velocity is calculated from the Navia-Stokes equation and the continuous equation for the filling element and the surface element. Here, the “pressure” is the pressure of the molten metal.

(B) Molten metal moving step In the molten metal moving step, the amount of fluid moving in a minute time is calculated from the VOF equation. Thereby, the filling degree of each cavity element is expressed by a filling rate (fluid rate). For example, when the filling rate is 0 or less, it is a void element, and when the filling rate> 0, it is a filling element. As described above, in this specification, a minute element (a cavity element having a filling rate larger than 0 and equal to or smaller than 1) filled with a molten metal is referred to as a “filling element”. Conversely, a minute element with a filling rate = 0 is referred to as a “gap element”.

(C) Element Flag Change Step In the element flag change step, for example, a void pressure (atmospheric pressure in the cavity) is applied to the surface element as a boundary condition.
These steps are repeated until the filling of the molten metal is completed. The completion of the filling is determined by examining the type of the cavity element and determining whether 90% or more of the cavity elements have become the filling elements. Alternatively, it may be determined when the filling time calculated in advance from the initial molten metal amount and the cavity volume is exceeded. At this time, when there is an unfilled element, the temperature of the unfilled element may be sequentially applied to the temperature of the adjacent element.

(2) Thermal stress analysis step and crack determination step The thermal stress analysis step is a step of sequentially calculating thermal stress acting on at least a part of the mold elements in accordance with the filling process of the filling analysis step. The crack determination step is a step of determining whether or not the calculated thermal stress exceeds at least a crack stress that is a determination index for occurrence of mold cracks.
In the present invention, first, in the thermal stress analysis step, the thermal stress generated in the mold during the filling of the molten metal is accurately grasped, and then in the crack determination step, the calculated thermal stress and the strength of the mold (cracking stress) and By comparing these, it is possible to accurately predict a mold crack that may occur in the mold.

  By the way, it is considered that local fractures caused by large stress acting at one time, such as mold cracking, often occur where the thermal stress becomes maximum, that is, where the maximum principal stress acts (the largest principal stress). Stress theory). Therefore, the thermal stress to be compared with the cracking stress is preferably the maximum principal stress acting on each mold element (particularly the maximum principal stress on the tension side). In this specification, this maximum principal stress is called the maximum thermal principal stress.

  Therefore, it is preferable that the thermal stress analysis step of the present invention further includes a maximum thermal principal stress calculation step for calculating a maximum thermal principal stress that is the maximum principal stress of the thermal stress acting on the mold element. In this case, the crack determination step is a step of determining whether or not the maximum thermal principal stress exceeds at least a crack stress that serves as an index of mold cracking.

(3) Temperature analysis step In the thermal stress analysis step, the thermal stress acting on the mold element is calculated, and the driving source of this thermal stress is mainly in the thermal expansion difference of each mold element. The difference in thermal expansion is greatly influenced by the temperature difference of each mold element. Therefore, it is preferable that the thermal stress analysis step is a temperature analysis step for sequentially calculating the thermal stress acting on the mold element based on the temperature of the mold element. Of course, the specific thermal stress itself is determined in consideration of the mold shape, the mold constraint conditions, the mold material, and the like in addition to the temperature of the mold element.

(4) Cracking stress (effective volume calculation step and cracking stress calculation step)
(A) In order to determine whether or not mold cracks occur, it is important how to set the crack stress to be compared with the thermal stress (particularly the maximum thermal principal stress) calculated in the thermal stress analysis step. In the case of molds, it is relatively easy to specify the yield stress, proof stress or tensile strength (breaking strength), high temperature properties, etc. of the metal constituting the mold based on known data or tensile test results. is there. Further, in the case of a mold, it is homogeneous as a whole, has a relatively stable behavior, and is less likely to cause mold cracks, so it is relatively easy to handle.
On the other hand, a caking type such as a sand mold has a large variation when viewed microscopically, and it is difficult to simply set a cracking stress as an index of mold cracking. For example, even if the strength of caking type is measured in advance by a tensile test or the like, cracks (fractures) often occur from local starting points (defects), and it is difficult to simply set a stable cracking stress. It is in.

Therefore, the present inventor conducted intensive research and succeeded in appropriately evaluating the caking type cracking stress by utilizing the Weibull distribution related to the strength and fracture of the brittle material. Specifically,
The cracking stress of the mold (caking mold) has a correlation with the effective volume, and as shown schematically in FIG.
The crack stress tends to decrease as the effective volume increases. The correlation between the cracking stress and the effective volume of the mold shown in FIG. 10 can be obtained by performing a tensile test or a bending test on the target mold. In addition, the effective volume is a volume integral value (triple integral value) obtained by Expression (1). By the way, the Weibull distribution itself is described in the literature [W. Weibull, J. et al. Appl. M
eh. Vol. 18 (1951), P293] and the like.
(Effective volume) = ∫V (σ / σ _max ) ^m dV (1)
V: integral volume (excess area in the mold)
σ: Maximum principal stress of the mold element in the integral volume V
σ _max : Maximum value of maximum principal stress σ
m: Weibull coefficient

Here, as an example, for example, consider the effective volume of a test piece having a square cross section. When m is the Weibull coefficient, w is the cross-sectional width of the test piece, t is the cross-sectional height of the test piece, and d is the distance between the fulcrums for the bending test, the effective volume is expressed as follows.
In the case of three-point bending (effective volume) = w · t · d / 2 (m + 1) ²
In the case of 4-point bending (effective volume) = w · t · d · (m + 3) / 6 (m + 1) ²

  As described above, in the present invention, the effective volume calculating step for calculating the effective volume determined based on the above formula (1) in the excess region in the mold where the thermal stress calculated in the thermal stress analysis step exceeds at least the reference stress. And a crack stress calculating step for calculating a crack stress based on the effective volume. These may be in the thermal stress analysis step or in the crack determination step.

(B) The excess region in the mold, which is the integration range when determining the effective volume here, is the reference stress in which the thermal stress acting on each mold element (especially the maximum thermal principal stress) is set for each type. This is a mold region exceeding (for example, tensile strength). Even if the reference stress is set to be constant, the excess region in such a mold fluctuates accordingly when the thermal stress acting on each mold element changes. Therefore, it is preferable that an excess region specifying step in the mold for specifying the excess region in the mold from the mold region in which the thermal stress is calculated is incorporated during the effective volume calculation step or before the effective volume calculation step.

Incidentally, if the excess area in the mold increases, the effective volume also increases accordingly, and the cracking stress correlated with the effective volume decreases. As a result, the probability that the thermal stress (maximum thermal principal stress) acting on each mold element exceeds the crack stress that serves as an index of mold cracking increases. That is, the probability that a mold crack will occur increases.
The effective volume calculation step (including the excess region identification step in the mold) and the crack stress calculation step are processed according to the thermal stress calculated in the thermal stress analysis step, so the actual calculation order is Apart from that, it can be said that it is included in the filling analysis step in terms of technical thought.

(5) Solidification analysis step The solidification analysis step is a step of sequentially calculating solidification processes in which the filled metal melt solidifies. The solidification analysis step is not necessarily essential when the focus is on the prediction of mold cracking during the filling analysis step as in the present invention, but it is generally included in the casting simulation.

  As shown in FIG. 9, for example, the solidification analysis process includes a solidification characteristic definition step and a solidification analysis step. In the solidification characteristic definition step, the solidification temperature, the solid phase ratio corresponding to the temperature, and the like are defined according to the type of molten metal. In the solidification analysis step, the calculation (step) is sequentially advanced at a predetermined minute reference time according to the solidification characteristics of the molten metal, and the solidification behavior of the molten metal is analyzed at each step. The analysis method itself in this case is not particularly limited. For example, energy conservation law is used as a basic expression for heat transfer analysis, and as a numerical solution of differential equations, known or commonly used methods such as forward Euler method, backward Euler method, Crank-Nicholson method, Runge-Kutta method, etc. An improved method is used. In addition, as a method for handling solidification, a known or commonly used method such as a temperature recovery method, an equivalent specific heat method, an enthalpy method, or a method obtained by improving them is used.

The present invention will be described more specifically with reference to examples.
<Manufacture of castings and confirmation of mold cracking>
In order to confirm the effectiveness of the casting simulation method according to the present invention, first, a hollow casting was actually manufactured and a position where a mold crack occurred was specified.

<Manufacture of mold>
A core (mold) made of a sand mold in which two kinds of particles shown in FIGS. 11 and 12 were hardened with a binder was manufactured. Specifically, first, 1.5% by mass of a phenol resin as a binder was added to No. 6 silica sand to prepare a mixed powder that was uniformly kneaded. The kneaded powder was put into a mold and pressed to obtain a powder compact having a shape generally shown in FIG. 11 or FIG. Furthermore, the powder compact was put into a heating furnace and heated at 250 ° C. for 60 seconds and fired to obtain a core shown in FIG. 11 or FIG. For convenience, the core shown in FIG. 11 is called a first core, and the core shown in FIG. 12 is called a second core.

  11 (a) shows the first core placed in the outer mold, FIG. 11 (b) is a front perspective view thereof, and FIG. 11 (c) is an A shown in FIG. 11 (b). It is -A sectional drawing. FIG. 12A shows the second core placed in the outer mold, and FIG. 12B is a cross-sectional view taken along line AA shown in FIG. Incidentally, the outer molds of FIGS. 11A and 12A are also sand molds similar to the above core.

<Preparation and pouring of molten metal>
A molten aluminum alloy (JIS AC4C) was prepared as a molten metal to be poured (filled) around each of the above-described cores.
The molten metal at a temperature of 700 ° C. was poured around the core shown in FIG. 11 (a) over 30 seconds without being pressurized from the gate shown in FIG. Further, the molten metal at a temperature of 700 ° C. was poured around the core shown in FIG. 12A without applying pressure from the gate shown in FIG.

<Observation of hollow casting>
Thus, a casting (first casting) that was gravity cast using the first core and a casting (second casting) that was gravity cast using the second core were obtained. After the cores were disintegrated and removed from each casting and the surface of each casting was lightly washed, the hollow interior formed by the cores was observed.

  As a result, in the first casting, the formation of burrs was confirmed at the “F” position shown in FIG. In the second casting, the formation of burrs was similarly confirmed at the position “F” shown in FIG. It is considered that these burrs were formed because cracks (mold cracks) occurred in the respective cores, and the molten metal poured into the gaps entered.

《Casting simulation》
The casting simulation method according to the present invention was performed, and the results were compared with the actual measurement results to evaluate the compatibility between them. The details will be described below.
(1) Model setting process The model setting process shown in FIG. 2 was performed using CAD data of the mold shown in FIGS. That is, a mold model used for casting simulation was formed (model formation step), and the mold model was divided into minute elements (element creation step). The division method was a mesh division with rectangular elements. Furthermore, each element was defined as a cavity element or a mold element (element definition step). A cavity element in the vicinity of the gate is selected, and a pressure inflow position is set to the selected cavity element (inflow position setting step).

(2) Filling analysis step and solidification analysis step Filling analysis and solidification analysis of the molten metal into the cavity element (void element) of the above-mentioned mold model are performed in consideration of actual casting conditions, as described above with reference to FIG. 6 (FIG. 7). And it carried out along each step shown in FIG. Completion of filling was judged when 90% or more of the cavity elements became filling elements, and completion of solidification was judged when the solid phase ratio of the cavity elements became 1. The simulation is completed when the solidification is completed.

  By the way, during the above-described filling analysis step, the thermal stress (maximum principal stress) acting on each mold element is sequentially calculated from the temperature obtained for each mold element corresponding to the first core and the second core (temperature). (Analysis step), and the maximum value among them was defined as the maximum thermal principal stress (maximum thermal principal stress calculation step). The maximum thermal principal stress thus determined was compared with the crack stress of each core (crack determination step). These steps were repeated until the molten metal was completely filled in the cavity element, and the presence / absence of occurrence of mold cracks and the occurrence position thereof were predicted.

(2) Cracking stress As the cracking stress of the core to be compared with the maximum thermal principal stress, in the case of the casting simulation of the first casting shown in FIG. The tensile strength (1.5-2.65 MPa) obtained by the tensile test was used for (corresponding to JIS Z2201 No. 5).

  On the other hand, in the case of the casting simulation of the second casting shown in FIG. 12, as shown in FIG. 8, the reference stress (0.2 MPa) is applied to the modeled second core based on the thermal stress obtained from the temperature analysis step. The excess region in the mold on which the thermal stress exceeding the above acts is identified (excess region identification step in the mold), and the effective volume is calculated based on the above-described formula (1) for the excess region in the mold (effective volume calculation step) ). Incidentally, the reference stress at this time was the lower limit (minimum value) of the tensile test strength.

  Next, based on the correlation (see FIG. 13) between the effective volume and cracking stress obtained in advance for the actual test piece manufactured in the same manner as the second core, The corresponding cracking stress (tensile strength) of the second core was identified. Note that the correlation between the effective volume and the cracking stress was obtained from a bending test and a tensile test performed while changing the size of the test piece.

<Evaluation>
(1) Casting simulation of the first casting When the casting simulation was performed for the first casting, the maximum thermal principal stress at the position F shown in FIG. 11A was 0.6 MPa or more, and the cracking stress (0 .33 MPa). This simulation result is well matched with the above-mentioned actual measurement result in which a mold crack occurs at the F position.

(2) Casting simulation of the second casting Even when the casting simulation is performed for the second mold, a prediction result that a mold crack occurs at the position F shown in FIG. 12 (a) is obtained. It was compatible.

  In addition, the effective volume and the maximum thermal principal stress (maximum thermal principal stress diagram) at the time of filling were obtained by performing the thermal stress analysis step of the casting simulation of the second casting. This was plotted superimposed on FIG. 13 showing the correlation (cracking stress diagram) between the effective volume and cracking stress (tensile strength of the mold).

As can be seen from the maximum thermal principal stress diagram obtained by this simulation, the maximum thermal principal stress and the effective volume increase as the molten metal filling process proceeds. In the crack determination step, when the maximum thermal principal stress that increases with the increase in the effective volume overlaps the tensile strength (cracking stress) of the mold determined by the effective volume at that time, it was determined as a mold crack.
Incidentally, the time when this mold crack occurred was 12 seconds after the pouring of the molten metal in the simulation. Since it takes 30 seconds from the pouring of the molten metal to filling, the generation time of cracking stress occurs during the filling of the molten metal. This agrees well with the knowledge obtained by the present inventor through various actual castings.

  As is clear from the above, it was confirmed that according to the casting simulation method according to the present invention, it is possible to predict mold cracks by reproducing actual experimental results with higher accuracy.

It is a main flowchart which shows the process sequence of the casting simulation method of this invention. It is a subflowchart which detailed the model setting process in the main flowchart. It is a schematic diagram which shows a mode that the type | mold model was divided | segmented into the microelement. It is a schematic diagram which shows a mode that the divided | segmented microelement was defined as a mold element or a cavity element. It is a figure which shows the basic equation used by the filling analysis. It is a subflowchart which explained the filling analysis process in the main flowchart in detail. It is the subflowchart which explained the filling analysis step in a filling analysis process in further detail. It is the subflowchart which explained in detail the thermal stress analysis step in a filling analysis process. It is a sub-flowchart which explained in detail the solidification analysis process in the main flow chart. It is the schematic which shows that the crack stress used as the parameter | index of a mold crack correlates with an effective volume. It is a figure which shows the shape and casting plan of the 1st core (mold) which concern on 1st Example to which the casting simulation of this invention is applied. It is a figure which shows the shape and casting plan of the 2nd core (mold) which concern on 2nd Example to which the casting simulation of this invention is applied. It is a figure which shows the actual correlation of effective volume-cracking stress, and the simulation result of effective volume-maximum thermal principal stress regarding 2nd Example.

Claims (7)

A model setting step for setting a mold model in which a mold constituting a cavity filled with molten metal is modeled on a coordinate system;
A casting simulation method comprising: a filling analysis step for sequentially calculating how the molten metal is filled into the cavities in the set mold model,
The model setting step includes a model formation step of forming the mold model by positioning it on a coordinate system, an element creation step of creating a large number of minute elements by finely dividing an area in the formed mold model, An element defining step of defining a microelement corresponding to a cavity region in the mold model as a cavity element among the created microelements and defining a microelement corresponding to the mold region in the mold model as a mold element; Have
The filling analysis step includes a filling analysis step for calculating a filling process in which the molten metal is sequentially filled into the cavity elements, and a thermal stress analysis step for sequentially calculating thermal stress acting on at least a part of the mold elements. A crack determination step for determining whether or not the calculated thermal stress exceeds at least a crack stress which is a determination index for occurrence of mold cracking; and an excess region in the mold where the calculated thermal stress exceeds at least a reference stress An effective volume calculating step for calculating an effective volume obtained based on the following mathematical formula (1), and a crack stress calculating step for calculating the cracking stress based on the effective volume, and at least occurrence of mold cracking. A casting simulation method characterized by being able to predict the presence or absence.
(Effective volume) = ∫ _V (σ / σ _max ) ^m dV (1)
V: integral volume (excess area in the mold)
σ: Maximum principal stress of the mold element in the integral volume V
σ _max : Maximum value of maximum principal stress σ
m: Weibull coefficient 2. The casting simulation method according to claim 1 , further comprising an excess region specifying step in the die that specifies an excess region in the die from the die region in which the thermal stress is calculated. The thermal stress analysis step further includes a maximum thermal principal stress calculation step for calculating a maximum thermal principal stress that is a maximum principal stress of thermal stress acting on the mold element,
The crack determination step, casting simulation method according to claim 1 or 2 said maximum thermal principal stress is determining whether exceeds at least the cracking stress. The thermal stress analysis step, casting simulation method according to any one of claims 1 to 3 thermal stress acting on the mold elements on the basis of the temperature of the mold element is a temperature analysis step of sequentially calculating. The casting simulation method according to any one of claims 1 to 4 , wherein the mold to be subjected to the thermal stress analysis step is a mold in which particles are hardened with a binder. The casting simulation method according to claim 5 , wherein the mold is a core. Casting simulation program casting simulation method cause the computer to function to according to any one of claims 1 to 6, characterized in that run.

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